R Interface to MiniZinc

Introduction

MiniZinc is a free and open-source constraint modeling language. Constraint satisfaction and discrete optimization problems can be formulated in a high-level modeling language. Models are compiled into an intermediate representation that is understood by a wide range of solvers. MiniZinc itself provides several solvers, for instance GeCode. The existing packages in R are not powerful enough to solve even mid-sized problems in combinatorial optimization.

There are implementations of an Interface to MiniZinc in Python like MiniZinc Python and pymzn and JMiniZinc for Java but such an interface does not exist for R.

This package provides an implementation of a very simple and easy to use interface for R that will help R users to solve optimization problems that couldn’t be solved with R till now.

It’s important to understand R6 classes before getting into the details. If you are not comfortable with R6, please go through this tutorial.

It would be nice to go through the tutorials on the MiniZinc website to understand more about MiniZinc. This is mainly for those who are interested in contributing to the package.

Installation

NOTE: This package is compatible with MiniZinc 2.5.5.

1. Build libminizinc

First, You need to download the libminizinc 2.5.5 release and build libminizinc library for MiniZinc to work properly.

Please follow these steps for Linux:

• Download the libminizinc 2.5.5 release from https://github.com/MiniZinc/libminizinc/releases.
• Extract the downloaded tar.gz or zip (for Windows) and name the extracted folder libminizinc.
• cd libminizinc/
  
• sudo sed -i '3 i set(CMAKE_POSITION_INDEPENDENT_CODE ON)' CMakeLists.txt
  
• sudo cmake CMakeLists.txt
  
• sudo make
  
• sudo make install

Similarly, build libminizinc on Windows (can use cygwin) and OSX.

If sed command doesn’t work for you, just add set(CMAKE_POSITION_INDEPENDENT_CODE ON) in the 3rd line (or any empty line in the starting) of CMakeLists.txt and follow the next steps.

2. Get Solver Binaries

Solvers are required to solve the MiniZinc models. The solvers currently supported by rminizinc are Chuffed, FindMUS and Gecode. Any solver can be selected based on the type of problem that is required to be solved.

Now download the solver binaries from the binary bundles at (https://www.minizinc.org/) to be able to solve the models and achieve full functionality of the package.

To get the solver binaries, the Users can download the MiniZinc binary bundles for Windows, MAC OS or Linux from https://www.minizinc.org/software.html and the provide the path to the bin folder of the MiniZinc bundle folder as the --with-bin argument. All the required solver binaries are present in that folder. The solver binary corresponding to Gecode will be fzn-gecode, FindMUS will be findMUS, Chuffed will be fzn-chuffed (.exe extentions will be there on Windows for eg. fzn-gecode.exe). Alternatively, if you don’t want to keep the MiniZinc bundle, you can copy the solver binaries to another folder and just provide the path to that folder with --with-bin.

Once these steps are over, you just need to re-install rminizinc by using

install.packages("rminizinc", configure.args="--with-mzn=/path/to/libminizinc --with-bin=/path/to/bin")

NOTE: Please don’t use \ at the end of the path given to --with-bin as it will cause some solver configuration issues.

Please note that if path arguments are not passed along with the installation (as --with-mzn), the default path /usr/local/lib for Linux and OSX, and C:/Program Files/ for Windows will be chosen but only if libminizinc in present in these default paths.

Getting Started

Load the library and the project root directory path

library(rminizinc)

## Loading required package: rjson

# load the PROJECT_DIRECTORY
data("proot")

# check if the library is present
data("config")
parse.next = FALSE
if(LIBMINIZINC_PATH == ""){
  warning("Please install libminizinc on your system!")
  parse.next = TRUE
}
# check if solver binaries are present
data("slvbin")
evaluate.next = FALSE
if(SOLVER_BIN == ""){
  warning("Please download solver binaries to solve the model")
  evaluate.next = TRUE
}

## Warning: Please download solver binaries to solve the model

# check if vignettes should be executed
if (!requireNamespace("rmarkdown") ||
!rmarkdown::pandoc_available("1.14")) {
     warning(call. = FALSE, "This vignette assumes that rmarkdown and pandoc
version 1.14 are available. These were not found. Older versions will not work.")
     knitr::knit_exit()
   }

Some Common CP Problems

Functions have been provided to directly solve some of the common constraint programming problems.

1. Knapsack problem

The knapsack problem is a problem in combinatorial optimization: Given a set of items, each with a weight and a value, determine the number of each item to include in a collection so that the total weight is less than or equal to a given limit and the total value is as large as possible. It derives its name from the problem faced by someone who is constrained by a fixed-size knapsack and must fill it with the most valuable items. The problem often arises in resource allocation where the decision makers have to choose from a set of non-divisible projects or tasks under a fixed budget or time constraint, respectively.

Here, n is the number of items, capacity is the total capacity of carrying weight, profit is the profit corresponding to each item and weight is the weight/size of each item. The goal is to maximize the total profit.

# knapsack problem
result = knapsack(n = 3, capacity = 9, profit = c(15,10,7), size = c(4,3,2))

## Error in mzn_eval(r_model = model): Please install libminizinc on your system and provide solver binaries!

cat(sprintf("The minizinc representation of the problem is:\n%s", result$model$mzn_string()))

## Error in sprintf("The minizinc representation of the problem is:\n%s", : object 'result' not found

cat(sprintf("The solutions returned by minizinc are:\n%s", result$solution$SOLUTION_STRING))

## Error in sprintf("The solutions returned by minizinc are:\n%s", result$solution$SOLUTION_STRING): object 'result' not found

# R representation of solutions
print(result$solution$SOLUTIONS)

## Error in print(result$solution$SOLUTIONS): object 'result' not found

2. Assignment problem

The assignment problem is a fundamental combinatorial optimization problem. In its most general form, the problem is as follows:

The problem instance has a number of agents and a number of tasks. Any agent can be assigned to perform any task, incurring some cost that may vary depending on the agent-task assignment. It is required to perform as many tasks as possible by assigning at most one agent to each task and at most one task to each agent, in such a way that the total cost of the assignment is minimized.

Here, n is the number of agents, m is the number of tasks and the profit(cost) is an m x n 2D array where each row corresponds to the cost of each task for that agent. Please provide the profit array as a 1D vector only as shown below:

# assignment problem
result  = assignment(n = 4, m = 5, cost = c(7,1,3,4,6,8,2,5,1,4,4,3,7,2,5,3,1,6,3,6))

## Error in mzn_eval(r_model = model): Please install libminizinc on your system and provide solver binaries!

cat(sprintf("The minizinc representation of the problem is:\n%s", result$model$mzn_string()))

## Error in sprintf("The minizinc representation of the problem is:\n%s", : object 'result' not found

cat(sprintf("The solutions returned by minizinc are:\n%s", result$solution$SOLUTION_STRING))

## Error in sprintf("The solutions returned by minizinc are:\n%s", result$solution$SOLUTION_STRING): object 'result' not found

# R representation of solutions
print(result$solution$SOLUTIONS)

## Error in print(result$solution$SOLUTIONS): object 'result' not found

Some other problem specific functions that are available are magic_series(), magic_square(), assignment_2(), and production_planning(). More problems will be added with version updates.

Parse a MiniZinc Model

A parser function mzn_parse has been implemented which can be used to detect possible syntax errors and get the smallest of details before the MiniZinc model is evaluated. The function returns the initialize Model R6 object.

Now, let’s solve a knapsack model:

# mzn file path

# load the PROJECT_DIRECTORY
data("proot")

"
NOTE: PROJECT_DIRECTORY is the project root directory path. This path is used for pointing to the mzn files provided with the package. If you have installed the package, PROJECT_DIRECTORY will initially point to tmp as the R package installation starts there. However, the next chunk of code will detect that and point to the installed rminizinc root folder. If you have installed rminizinc in some other location please make the PROJECT_DIRECTORY point to that folder.
"

mzn_local_location = "/inst/extdata/mzn_examples/"

if(grepl( "tmp", PROJECT_DIRECTORY, fixed = TRUE)){
  message("PROJECT_DIRECTORY is not a correct location anymore. Now PROJECT_DIRECTORY
          is pointing to the location of installed rminizinc package. Please note that
          the PROJECT_DIRECTORY stored in proot.RData is still the ")
  PROJECT_DIRECTORY = paste0(.libPaths()[1], "/rminizinc")
  mzn_local_location = "/extdata/mzn_examples/"
}

## PROJECT_DIRECTORY is not a correct location anymore. Now PROJECT_DIRECTORY
##           is pointing to the location of installed rminizinc package. Please note that
##           the PROJECT_DIRECTORY stored in proot.RData is still the

mzn_path = paste0(PROJECT_DIRECTORY, mzn_local_location, "knapsack/knapsack_0.mzn")

# returns the R equivalent of a MiniZinc model
parsedModel = rminizinc:::mzn_parse(mzn_path = mzn_path)

cat(sprintf("The current model is:\n%s", parsedModel$mzn_string()))

## [1] "\nNOTE: PROJECT_DIRECTORY is the project root directory path. This path is used for pointing to the mzn files provided with the package. If you have installed the package, PROJECT_DIRECTORY will initially point to tmp as the R package installation starts there. However, the next chunk of code will detect that and point to the installed rminizinc root folder. If you have installed rminizinc in some other location please make the PROJECT_DIRECTORY point to that folder.\n"
## The current model is:
## int: n;
## 
## set of int: OBJ = (1 .. n);
## 
## int: capacity;
## 
## array[OBJ] of int: profit;
## 
## array[OBJ] of int: size;
## 
## array[OBJ] of var int: x;
## 
## constraint forall([(x[i] >= 0) | i in OBJ ]);
## 
## constraint (sum([(size[i] * x[i]) | i in OBJ ]) <= capacity);
## 
## solve  maximize sum([(profit[i] * x[i]) | i in OBJ ]);
## 
## include "solver_redefinitions.mzn";
## 
## include "stdlib.mzn";

Look at the contents of parseObj for more understanding of the model.

Find the Missing Parameters

The missing parameters can be obtained using get_missing_pars()

missingPars = get_missing_pars(model = parsedModel)
print(missingPars)

## [1] "n"        "capacity" "profit"   "size"

Set Values of Missing Parameters

The missing parameters can be directly provided a value instead of using a data (dzn) file to solve. The values can be provided as a named list of the parameters. The data will be entered according to the data file as shown below:

n = 3;
capacity = 9;
profit = [15,10,7];
size = [4,3,2];

# Int$new() for creating a new integer in model
# Array$new() for creating an array
# intExpressions() to create a sequence of integers (useful for array)
pVals = list(Int$new(3), Int$new(9), Array$new(c(Int$new(15), Int$new(10), Int$new(7)),
                                               dimranges = c(IntSetVal$new(1, 3))),
            Array$new(intExpressions(c(4,3,2)), dimranges = c(IntSetVal$new(1, 3))))

names(pVals) = missingPars

model = set_params(model = parsedModel, modData = pVals)

# check ?Model for more details
cat(sprintf("The updated model is:\n %s", model$mzn_string()))

## The updated model is:
##  int: n = 3;
## 
## set of int: OBJ = (1 .. n);
## 
## int: capacity = 9;
## 
## array[OBJ] of int: profit = [15, 10, 7];
## 
## array[OBJ] of int: size = [4, 3, 2];
## 
## array[OBJ] of var int: x;
## 
## constraint forall([(x[i] >= 0) | i in OBJ ]);
## 
## constraint (sum([(size[i] * x[i]) | i in OBJ ]) <= capacity);
## 
## solve  maximize sum([(profit[i] * x[i]) | i in OBJ ]);
## 
## include "solver_redefinitions.mzn";
## 
## include "stdlib.mzn";

Solve the Model

The function mzn_eval() is used to evaluate a MiniZinc model and returns the solution string and a list of solutions if they were parsed without any error by the function sol_parse() otherwise it returns the solution string and an appropriate error. The parsed solutions are a named list where elements are of type OBJ$SOLUTIONS$SOLUTION<n>$<VARIABLE_NAME>. The optimal solution if found can be accessed using OBJ$SOLUTIONS$OPTIMAL_SOLUTION and the best solution can be accessed using OBJ$SOLUTIONS$BEST_SOLUTION. More details about the functions can be obtained using ?mzn_eval and ?sol_parse.

The solver name of the solver that should be used to solve the model needs to be specified by the user (default is “Gecode”) and the lib_path (path of the solver configuration files) is by default provided but a custom path can be provided the user in case it is required. The model must be provided as one and only one of R6 Model object, mzn_path i.e. path of mzn file or model_string i.e. the string representation of the model. If the user wishes to provide a data file, it’s path can be provided to the argument dznpath. A time limit (in ms) can also be provided to the argument time_limit. (default is 300000 ms). Other possible command line options can be provided as c("a", .., "n") to other_cl_options. Changing the default solution outputs will result in parsing errors and the solutions will not be parsed correctly to R but the solution string will be returned.

A sample knapsack problem has been solved below::

# R List object containing the solutions
solObj = rminizinc:::mzn_eval(r_model = model)

## Error in rminizinc:::mzn_eval(r_model = model): Please install libminizinc on your system and provide solver binaries!

# get all the solutions
print(solObj$SOLUTIONS)

## Error in print(solObj$SOLUTIONS): object 'solObj' not found

The above example is taken from minizinc-examples.

NOTE: The output formatting in the mzn files will be automatically removed in the parsed R model.

Items

A MiniZinc model is made of several items namely, Variables, Constraints, Solve Items, Assignments,etc. which are described below.

# int: a;
a = VarDeclItem$new(decl = IntDecl(name = "a", kind = "par"))
a$c_str()

## [1] "int: a;\n"

# var int: a;
a = VarDeclItem$new(decl = IntDecl(name = "a", kind = "var"))
a$c_str()

## [1] "var int: a;\n"

# 0..3: a;
a = VarDeclItem$new(decl = IntDecl(name = "a", kind = "par", 
                                   domain = Set$new(IntSetVal$new(imin = 0, imax = 3))))
a$c_str()

## [1] "0..3: a;\n"

# set of int: b = 1..5;
b = VarDeclItem$new(decl = IntSetDecl(name = "b", kind = "par", 
                                      value = Set$new(IntSetVal$new(imin = 1, imax = 5))))
b$c_str()

## [1] "set of int: b = 1..5;\n"

# set of int: b = {1, 3, 5, 7, 9};
b = VarDeclItem$new(decl = IntSetDecl(name = "b", kind = "par", 
                                      value = Set$new(val = intExpressions(c(1, 3, 5 ,7 ,9)))))
b$c_str()

## [1] "set of int: b = {1, 3, 5, 7, 9};\n"

# int: IND = 3;
a = VarDeclItem$new(decl = IntDecl(name = "IND", kind = "par", value = 3))
a$c_str()

## [1] "int: IND = 3;\n"

val = a$getDecl()$getValue()$getIntVal()
sprintf("The value of a is: %s", val)

## [1] "The value of a is: 3"

# array[IND] of int: p = [15,10,7];
p = VarDeclItem$new(decl = IntArrDecl(name = "p", kind = "par", ind = c(a$getId()),
                                  value = Array$new(exprVec = intExpressions(c(15, 10, 7)),
                                                    dimranges = c(IntSetVal$new(1, val))),
                                        ndim = 1))
p$c_str()

## [1] "array[IND] of int: p = [15, 10, 7];\n"

# array[IND] of int: p = [|1, 2, 3 | 4, 5, 6 | 7, 8, 9|];
# Array can only be used to provide 1D and 2D array values
array_value = Array$new(exprVec = c(intExpressions(c(1,2,3)), intExpressions(c(4,5,6)),                                                                    intExpressions(c(7,8,9))),
                                        dimranges = c(IntSetVal$new(1, val), IntSetVal$new(1, val)))
p = VarDeclItem$new(decl = IntArrDecl(name = "p", kind = "par", ind = c(a$getId(), a$getId()),
                    value = array_value, ndim = 2))
cat(p$c_str())

## array[IND, IND] of int: p = [|1, 2, 3,
## |4, 5, 6,
## |7, 8, 9
## |];

## Recommended way to provide array values
array_value = Call$new(fnName = "array2d", args = c(a$getId(), a$getId(),
                                                    intExpressions(c(1,2,3)), intExpressions(c(4,5,6)),                                                                    intExpressions(c(7,8,9))))
p$getDecl()$setValue(array_value)
cat(p$c_str())

## array[IND, IND] of int: p = array2d(IND, IND, 1, 2, 3, 4, 5, 6, 7, 8, 9);

# var int a; constraint a < 10;

# var int a;
a = VarDeclItem$new(decl = IntDecl(name = "a", kind = "var"))
a$c_str()

## [1] "var int: a;\n"

# a < 10;
exp = BinOp$new(lhs = a$getId(), binop = "<", rhs = Int$new(10))
exp$c_str()

## [1] "(a < 10)"

# constraint a < 10;
cnst = ConstraintItem$new(e = exp)
cnst$c_str()

## [1] "constraint (a < 10);\n"

# solve satisfy;
s = SolveItem$new(solve_type = "satisfy")
s$c_str()

## [1] "solve  satisfy;\n"

# var int: sum; 
sum = IntDecl(name = "sum", kind = "var")
sum$c_str()

## [1] "var int: sum"

# minimize sum;
s = SolveItem$new(solve_type = "minimize", e = sum$getId())
s$c_str()

## [1] "solve  minimize sum;\n"

# maximize sum;
s = SolveItem$new(solve_type = "maximize", e = sum$getId())
s$c_str()

## [1] "solve  maximize sum;\n"

Expressions

Given below are some examples of commonly used MiniZinc expressions. Check examples of some other expressions in the package manual.

# always provide ind (array index) in c() or list()
a = VarDeclItem$new(decl = IntArrDecl(name = "a", kind = "var",ind = c(Int$new(4)),ndim = 1))
a$c_str()

## [1] "array[4] of var int: a;\n"

iter = IntDecl(name = "i", kind = "par")
generator = Generator$new(IN = a$getId(), decls = list(iter))
bop = BinOp$new(lhs = ArrayAccess$new(v = a$getId(),  args= list(generator$getDecl(1)$getId())),
                                                             binop = ">=", rhs = Int$new(0))

comprehension = Comprehension$new(generators = list(generator), body = bop, set = FALSE)
comprehension$c_str()

## [1] "[(a[i] >= 0) | i in a ]"

cl = Call$new(fnName = "forall", args = list(comprehension))
cl$c_str()

## [1] "forall([(a[i] >= 0) | i in a ])"

# if x < 0 then -1 elseif x > 0 then 1 else 0 endif

# var int:x;
x = IntDecl(name = "x", kind = "var")
x$c_str()

## [1] "var int: x"

bop1 = BinOp$new(lhs = x$getId(), binop = "<", rhs = Int$new(0))
bop1$c_str()

## [1] "(x < 0)"

uop = UnOp$new(args = c(Int$new(1)), op = "-")
uop$c_str()

## [1] "-(1)"

bop2 = BinOp$new(lhs = x$getId(), binop = "<", rhs = Int$new(0))
bop2$c_str()

## [1] "(x < 0)"

exp = Ite$new(ifs = c(bop1, bop2), thens = c(uop, Int$new(1)), Else = Int$new(0))
exp$c_str()

## [1] "if ((x < 0)) then (-(1)) elseif ((x < 0)) then (1) else (0) endif"

# let { int: x = 3; int: y = 4; } in x + y
x = VarDeclItem$new(IntDecl(name = "x", kind = "par", value = 3))
y = VarDeclItem$new(IntDecl(name = "y", kind = "par", value = 4))
bop = BinOp$new(lhs = x$getId(), binop = "+", rhs = y$getId())
let = Let$new(let = c(x,y), body = bop)
cat(let$c_str())

## let {int: x = 3;
## int: y = 4;
## } in (x + y)

Create Items Using Strings

The strings containing MiniZinc syntax of items can be directly supplied to the constructors to initialize the objects. If strings are supplied, no other argument should be supplied to any of the Item classes except for AssignItem where you need to provided the associated variable declaration for the assignment.

declItem = VarDeclItem$new(mzn_str = "set of int: WORKSHEET = 0..worksheets-1;")
sprintf("Is this a parameter? %s", declItem$getDecl()$isPar())
sprintf("Is this a set? %s", declItem$getDecl()$ti()$type()$isSet())
sprintf("Base type of set: %s", declItem$getDecl()$ti()$type()$bt())
sprintf("Name: %s", declItem$getId()$getName())
sprintf("Value: %s", declItem$getDecl()$getValue()$c_str())

## [1] "Is this a parameter? TRUE"
## [1] "Is this a set? TRUE"
## [1] "Base type of set: int"
## [1] "Name: WORKSHEET"
## [1] "Value: (0 .. (worksheets - 1))"

CstrItem = ConstraintItem$new(mzn_str = "constraint forall (i in PREC)
                  (let { WORKSHEET: w1 = preceeds[i];
                             WORKSHEET: w2 = succeeds[i]; } in
                   g[w1] * e[w1] <= d[w2] + days * (1 - g[w2]));")
sprintf("Expression involved: %s", CstrItem$getExp()$c_str())
sprintf("Call function name: %s", CstrItem$getExp()$getName())
sprintf("Number of Arguments: %s", CstrItem$getExp()$nargs())
sprintf("Class of Argument: %s",  class(CstrItem$getExp()$getArg(1))[1])
sprintf("Number of Generators: %s", CstrItem$getExp()$nargs())
sprintf("Generator: %s", CstrItem$getExp()$getArg(1)$getGen(1)$c_str())
sprintf("Comprehension body: %s", CstrItem$getExp()$getArg(1)$getBody()$c_str())

## [1] "Expression involved: forall([let {WORKSHEET: w1 = preceeds[i];\nWORKSHEET: w2 = succeeds[i];\n} in ((g[w1] * e[w1]) <= (d[w2] + (days * (1 - g[w2])))) | i in PREC ])"
## [1] "Call function name: forall"
## [1] "Number of Arguments: 1"
## [1] "Class of Argument: Comprehension"
## [1] "Number of Generators: 1"
## [1] "Generator: i in PREC "
## [1] "Comprehension body: let {WORKSHEET: w1 = preceeds[i];\nWORKSHEET: w2 = succeeds[i];\n} in ((g[w1] * e[w1]) <= (d[w2] + (days * (1 - g[w2]))))"

SlvItem = SolveItem$new(mzn_str = "solve 
    :: int_search(
        [ if j = 1 then g[import_first[i]] else -d[import_first[i]] endif  | i in 1..worksheets, j in 1..2], 
        input_order, indomain_max, complete)
    maximize objective;")
sprintf("Objective: %s", SlvItem$getSt())
cat(sprintf("Annotation: %s", SlvItem$getAnn()$c_str()))

## [1] "Objective: maximize"
## Annotation:  :: int_search([if ((j = 1)) then (g[import_first[i]]) else (-(d[import_first[i]])) endif | i in (1 .. worksheets) , j in 1..2 ], input_order, indomain_max, complete)

fnItem = FunctionItem$new(mzn_str = "predicate nonoverlap(var int:s1, var int:d1,
                     var int:s2, var int:d2)=
          s1 + d1 <= s2 \\/ s2 + d2 <= s1;")
sprintf("Function name: %s", fnItem$name())
sprintf("No of function declarations: %s", length(fnItem$getDecls()))
sprintf("Function expression: %s", fnItem$getBody()$c_str())

## [1] "Function name: nonoverlap"
## [1] "No of function declarations: 4"
## [1] "Function expression: (((s1 + d1) <= s2) \\/ ((s2 + d2) <= s1))"

iItem = IncludeItem$new(mzn_str = "include \"cumulative.mzn\" ;")
sprintf("Included mzn name: %s", iItem$getmznName())

## [1] "Included mzn name: solver_redefinitions.mzn"

Create a Model from Scratch

Let’s compute the base of a right angled triangle given the height and hypotenuse. The Pythagoras theorem says that In a right-angled triangle, the square of the hypotenuse side is equal to the sum of squares of the other two sides i.e \(a² + b² = c²\). The theorem give us three functions, \(c = \sqrt{(a² + b²)}\), \(a = \sqrt(c² - b²)\) and \(b = \sqrt(c² - a²)\).

MiniZinc Representation of the model:

int: a = 4;
int: c = 5;
var int: b;

constraint b>0;
constraint a² + b² = c²;

solve satisfy;

a = IntDecl(name = "a", kind = "par", value = 4)
c = IntDecl(name = "c", kind = "par", value = 5)
b = IntDecl(name = "b", kind = "var")

# declaration items
a_item = VarDeclItem$new(decl = a)
b_item = VarDeclItem$new(decl = b)
c_item = VarDeclItem$new(decl = c)

# b > 0 is a binary operation
b_0 = BinOp$new(lhs = b$getId(), binop = ">", rhs = Int$new(0))
constraint1  = ConstraintItem$new(e = b_0)

# a ^ 2 is a binary operation
# a$getId() gives the variable identifier
a_2 = BinOp$new(lhs = a$getId(), binop = "^", Int$new(2))
b_2 = BinOp$new(lhs = b$getId(), binop = "^", Int$new(2))
a2_b2 = BinOp$new(lhs = a_2, binop = "+", rhs = b_2)
c_2 = BinOp$new(lhs = c$getId(), binop = "^", Int$new(2))
a2_b2_c2 = BinOp$new(lhs = a2_b2, binop = "=", rhs = c_2)
constraint2  = ConstraintItem$new(e = a2_b2_c2)

solve  = SolveItem$new(solve_type = "satisfy")

model = Model$new(items = c(a_item, b_item, c_item, constraint1, constraint2, solve))

cat(model$mzn_string())

## int: a = 4;
## 
## var int: b;
## 
## int: c = 5;
## 
## constraint (b > 0);
## 
## constraint (((a ^ 2) + (b ^ 2)) = (c ^ 2));
## 
## solve  satisfy;

Delete Items and Expressions

All the Item and Expression classes have a delete() function which is used to delete the objects from everywhere in the model. Note that the objects will be deleted from all the models present in the environment from where the delete() function is called. An example to demonstrate the same is shown below:

# delete the item a i.e declaration of a 
a$delete()
cat(model$mzn_string())

## var int: b;
## 
## int: c = 5;
## 
## constraint (b > 0);
## 
## constraint (((a ^ 2) + (b ^ 2)) = (c ^ 2));
## 
## solve  satisfy;

Modify Variable Domain

vd = VarDomainDecl(name = "n", dom = Set$new(IntSetVal$new(imin = 1, imax = 2)))
sprintf("The current declaration is: %s", vd$c_str())
vd$setDomain(Set$new(IntSetVal$new(imin = 3, imax = 5)))
sprintf("The modified declaration is: %s", vd$c_str())

## [1] "The current declaration is: var 1..2: n"
## [1] "The modified declaration is: var 3..5: n"

Modify Constraints

There are various getter and setter functions for the expression classes that can be used to modify existing constraints. For example:

vItem = VarDeclItem$new(mzn_str = "set of int: a = {1, 2, 3, 4};") 
cItem = ConstraintItem$new(mzn_str = "constraint sum(a) < 10;")
sprintf("The current constraint is: %s", cItem$c_str())
cItem$setExp(BinOp$new(lhs = Call$new(fnName = "max", args = list(vItem$getDecl()$getId())),
                       binop = "<", rhs = Int$new(10)))
sprintf("The modified constraint is: %s", cItem$c_str())

## [1] "The current constraint is: constraint (sum(a) < 10);\n"
## [1] "The modified constraint is: constraint (max(a) < 10);\n"